1. Field of the Invention
This invention relates to a finder optical system for use in photographic cameras, video cameras, etc.
2. Description of the Related Art
In a conventional finder optical system disposed independently of a photographing optical system, it is common that lenses are fixed, irrespective of an object distance, with no focusing function. With such a conventional finder optical system, a photographer, when looking through a finder, unconsciously corrects the positional shift of an aerial image formed through a finder objective optical system by virtue of the focus adjusting function of his eye. In this case, an apparent object distance L' where he sees through the finder, namely a diopter D, is determined by a finder angular magnification 7 and an object distance L and is given by EQU D=1/L'=1/L.times..gamma..sup.2 [1/m] (1)
Since the finder angular magnification .gamma. of a camera is generally .gamma.&lt;1, a change of diopter with changes in the object distance is less than that in a real field of view, and thus there is no problem that the aerial image is out of focus.
With recent development of compact zoom cameras with high variable magnification, however, the angular magnification .gamma. of the finder optical system sometimes becomes larger than 1. In this case, the change of diopter with changes in the object distance is greater than that in the real field of view. This change cannot be corrected by the focus adjusting function of the eye, with the resultant problem that an object is not brought into focus and is hard to see.
Furthermore, it is well known that in the camera in which the finder optical system is disposed independently of the photographing optical system, parallax is produced between the finder and photographing optical systems and thus it is difficult to correctly indicate a photographic range over the entire object distance on the image plane of the finder.
For means of solving the foregoing two problems, it is known that Japanese Patent Preliminary Publication No. Hei 1-197727 discloses the technique that a part of lenses constituting the finder optical system is moved along the optical axis and in a direction perpendicular thereto, based on object distance data, to correct for diopter and parallax at the same time.
According to this technique, however, when the part of lenses constituting the finder optical system is moved along the optical axis, the finder magnification will change and the angle of view of incidence will also change. In the photographing optical system, in which focusing is attended by a change in angle of view, a field factor which is the tangent ratio of the half angle of view of incidence on the photographing optical system to that on the finder optical system will vary widely if the amount of change in angle of view dose not coincide. Consequently, the problem that results is that even though the photographing optical system is made to coincide with the image plane of the finder by correction for diopter, unwanted objects lying outside the field of view of the finder will be photographed, or conversely, objects inside the field of view will not be photographed.
For compact zoom cameras, downsizing of the finder itself is also designed, which is disclosed, for example, by Japanese Patent Preliminary Publication No. Ilei 4-53914. This technique is that the power of each lens of an objective unit is increased and a zoom movement is limited so that a reflecting member for erecting an image can be inserted into an objective path and the size of the entire finder optical system is reduced. The increase of each power of the objective unit, however, may change the properties of the optical system (diopter, parallax, and imaging performance) when the lens unit moves. If, therefore, the method of correcting for diopter and parallax at the same time, set forth in Publication No. Hei 1-197727 previously mentioned, is applied, the lens movement from the infinite distance to the minimum distance will be limited and a complicated driving mechanism will be required to secure the accuracy of correction for diopter and parallax suitable for practical use. This causes the problem of raising the manufacturing cost of the camera. Moreover, because of lens decentering required for correction for parallax, the deterioration of imaging performance sometimes becomes pronounced.